Vector Group of Transformer


Three phase transformer consists of three sets of primary windings, one for each phase, and three sets of secondary windings wound on the same iron core. Separate single-phase transformers can be used and externally interconnected to yield the same results as a 3-phase unit.

The primary windings are connected in one of several ways. The two most common configurations are the delta, in which the polarity end of one winding is connected to the non-polarity end of the next, and the star, in which all three non-polarities (or polarity) ends are connected together. The secondary windings are connected similarly. This means that a 3-phase transformer can have its primary and secondary windings connected the same (delta-delta or star-star), or differently (delta-star or star-delta).

It’s important to remember that the secondary voltage waveforms are in phase with the primary waveforms when the primary and secondary windings are connected the same way. This condition is called “no phase shift.” But when the primary and secondary windings are connected differently, the secondary voltage waveforms will differ from the corresponding primary voltage waveforms by 30 electrical degrees. This is called a 30 degree phase shift. When two transformers are connected in parallel, their phase shifts must be identical; if not, a short circuit will occur when the transformers are energized.”

 Basic Idea of Winding:

  • An ac voltage applied to a coil will induce a voltage in a second coil where the two are linked by a magnetic path. The phase relationship of the two voltages depends upon which ways round the coils are connected. The voltages will either be in-phase or displaced by 180 deg
  • When 3 coils are used in a 3 phase transformer winding a number of options exist. The coil voltages can be in phase or displaced as above with the coils connected in star or delta and, in the case of a star winding, have the star point (neutral) brought out to an external terminal or not.

             Six Ways to wire Star Winding:


            Six Ways to wire Delta Winding:



  • An ac voltage applied to a coil will induce a voltage in a second coil where the two are linked by a magnetic path.  The phase relationship of the two voltages depends upon which way round the coils are connected.  The voltages will either be in-phase or displaced by 180 deg.
  • When 3 coils are used in a 3 phase transformer winding a number of options exist.  The coil voltages can be in phase or displaced as above with the coils connected in star or delta and, in the case of a star winding, have the star point (neutral) brought out to an external terminal or not.


  • When Pair of Coil of Transformer have same direction than voltage induced in both coil are in same direction from one end to other end.
  • When two coil have opposite winding direction than Voltage induced in both coil are in opposite direction.

Winding connection designations:

  • First Symbol: for High Voltage: Always capital letters.
  •  D=Delta, Y=Star, Z=Interconnected star, N=Neutral
  • Second Symbol: for Low voltage: Always Small letters.
  •  d=Delta, y=Star, z=Interconnected star, n=Neutral.
  • Third Symbol: Phase displacement expressed as the clock hour number (1,6,11)
  • Example – Dyn11
    Transformer has a delta connected primary winding (D) a star connected secondary (y) with the star point brought out (n) and a phase shift of 30 deg leading (11).
  • The point of confusion is occurring in notation in a step-up transformer. As the IEC60076-1 standard has stated, the notation is HV-LV in sequence. For example, a step-up transformer with a delta-connected primary, and star-connected secondary, is not written as ‘dY11’, but ‘Yd11’. The 11 indicates the LV winding leads the HV by 30 degrees.
  • Transformers built to ANSI standards usually do not have the vector group shown on their nameplate and instead a vector diagram is given to show the relationship between the primary and other windings.

Vector Group of Transformer:

  • The three phase transformer windings can be connected several ways. Based on the windings’ connection, the vector group of the transformer is determined.
  • The transformer vector group is indicated on the Name Plate of transformer by the manufacturer.
    The vector group indicates the phase difference between the primary and secondary sides, introduced due to that particular configuration of transformer windings connection.
  • The Determination of vector group of transformers is very important before connecting two or more transformers in parallel. If two transformers of different vector groups are connected in parallel then phase difference exist between the secondary of the transformers and large circulating current flows between the two transformers which is very detrimental.

Phase Displacement between HV and LV Windings:

  •  The vector for the high voltage winding is taken as the reference vector. Displacement of the vectors of other windings from the reference vector, with anticlockwise rotation, is represented by the use of clock hour figure.
  • IS: 2026 (Part 1V)-1977 gives 26 sets of connections star-star, star-delta, and star zigzag, delta-delta, delta star, delta-zigzag, zigzag star, zigzag-delta. Displacement of the low voltage winding vector varies from zero to -330° in steps of -30°, depending on the method of connections.
  • Hardly any power system adopts such a large variety of connections. Some of the commonly used connections with phase displacement of 0, -300, -180″ and -330° (clock-hour setting 0, 1, 6 and 11).
  • Symbol for the high voltage winding comes first, followed by the symbols of windings in diminishing sequence of voltage. For example a 220/66/11 kV Transformer connected star, star and delta and vectors of 66 and 11 kV windings having phase displacement of 0° and -330° with the reference (220 kV) vector will be represented As Yy0 – Yd11.
  • The digits (0, 1, 11 etc) relate to the phase displacement between the HV and LV windings using a clock face notation. The phasor representing the HV winding is taken as reference and set at 12 o’clock. Phase rotation is always anti-clockwise. (International adopted).
  • Use the hour indicator as the indicating phase displacement angle. Because there are 12 hours on a clock, and a circle consists out of 360°, each hour represents 30°.Thus 1 = 30°, 2 = 60°, 3 = 90°, 6 = 180° and 12 = 0° or 360°.
  • The minute hand is set on 12 o’clock and replaces the line to neutral voltage (sometimes imaginary) of the HV winding. This position is always the reference point.
  • Example:
  • Digit 0 =0° that the LV phasor is in phase with the HV phasor
    Digit 1 =30° lagging (LV lags HV with 30°) because rotation is anti-clockwise.
  • Digit 11 = 330° lagging or 30° leading (LV leads HV with 30°)
  • Digit 5 = 150° lagging (LV lags HV with 150°)
  • Digit 6 = 180° lagging (LV lags HV with 180°)
  • When transformers are operated in parallel it is important that any phase shift is the same through each. Paralleling typically occurs when transformers are located at one site and connected to a common bus bar (banked) or located at different sites with the secondary terminals connected via distribution or transmission circuits consisting of cables and overhead lines.

Phase Shift (Deg)






30 lag




60 lag




120 lag




150 lag




180 lag




150 lead




120 lead




60 lead




30 lead








  •  The phase-bushings on a three phase transformer are marked either  ABC, UVW or 123 (HV-side capital, LV-side small letters). Two winding, three phase transformers can be divided into four main categories
Group O’clock TC
Group I 0 o’clock, 0° delta/delta, star/star
Group II 6 o’clock, 180° delta/delta, star/star
Group III 1 o’clock, -30° star/delta, delta/star
Group IV 11 o’clock, +30° star/delta, delta/star
Minus indicates LV lagging HV, plus indicates LV leading HV

 Clock Notation: 0

Clock Notation : 1

 Clock Notation: 2

Clock Notation: 4

Clock Notation: 5

Clock Notation: 6

Clock Notation: 7

Clock Notation: 11

 Points to be consider while Selecting of Vector Group:

  • Vector Groups are the IEC method of categorizing the primary and secondary winding configurations of 3-phase transformers. Windings can be connected as delta, star, or interconnected-star (zigzag). Winding polarity is also important, since reversing the connections across a set of windings affects the phase-shift between primary and secondary. Vector groups identify the winding connections and polarities of the primary and secondary. From a vector group one can determine the phase-shift between primary and secondary.
  • Transformer vector group depends upon
    1. Removing harmonics: Dy connection – y winding nullifies 3rd harmonics, preventing it to be reflected on delta side.
    2. Parallel operations: All the transformers should have same vector group & polarity of the winding.
    3. Earth fault Relay: A Dd transformer does not have neutral. to restrict the earth faults in such systems, we may use zig zag wound transformer to create a neutral along with the earth fault relay..
    4. Type of Non Liner Load: systems having different types of harmonics & non linear Types of loads e.g. furnace heaters ,VFDS etc for that we may use Dyn11, Dyn21, Dyn31 configuration, wherein, 30 deg. shifts of voltages nullifies the 3rd harmonics to zero in the supply system.
    5. Type of Transformer Application: Generally for Power export transformer i.e. generator side is connected in delta and load side is connected in star. For Power export import transformers i.e. in Transmission Purpose Transformer star star connection may be preferred by some since this avoids a grounding transformer on generator side and perhaps save on neutral insulation. Most of systems are running in this configuration. May be less harmful than operating delta system incorrectly. Yd or Dy connection is standard for all unit connected generators.
    6. There are a number of factors associated with transformer connections and may be useful in designing a system, and the application of the factors therefore determines the best selection of transformers. For example:

For selecting Star Connection:

  • A star connection presents a neutral. If the transformer also includes a delta winding, that neutral will be stable and can be grounded to become a reference for the system. A transformer with a star winding that does NOT include a delta does not present a stable neutral.
  • Star-star transformers are used if there is a requirement to avoid a 30deg phase shift, if there is a desire to construct the three-phase transformer bank from single-phase transformers, or if the transformer is going to be switched on a single-pole basis (ie, one phase at a time), perhaps using manual switches.
  • Star-star transformers are typically found in distribution applications, or in large sizes interconnecting high-voltage transmission systems. Some star-star transformers are equipped with a third winding connected in delta to stabilize the neutral.

For selecting Delta Connection:

  • A delta connection introduces a 30 electrical degree phase shift.
  • A delta connection ‘traps’ the flow of zero sequence currents.

For selecting Delta-Star Connection:

  • Delta-star transformers are the most common and most generally useful transformers.
  • Delta-delta transformers may be chosen if there is no need for a stable neutral, or if there is a requirement to avoid a 30 electrical degree phase shift. The most common application of a delta-delta transformer is as tan isolation transformer for a power converter.

For selecting Zig zag Connection:

  • The Zig Zag winding reduces voltage unbalance in systems where the load is not equally distributed between phases, and permits neutral current loading with inherently low zero-sequence impedance. It is therefore often used for earthing transformers.
  • Provision of a neutral earth point or points, where the neutral is referred to earth either directly or through impedance. Transformers are used to give the neutral point in the majority of systems. The star or interconnected star (Z) winding configurations give a neutral location. If for various reasons, only delta windings are used at a particular voltage level on a particular system, a neutral point can still be provided by a purpose-made transformer called a ‘neutral earthing.

 For selecting Distribution Transformer:

  •  The first criterion to consider in choosing a vector group for a distribution transformer for a facility is to know whether we want a delta-star or star-star. Utilities often prefer star-star transformers, but these require 4-wire input feeders and 4-wire output feeders (i.e. incoming and outgoing neutral conductors).
  • For distribution transformers within a facility, often delta-star are chosen because these transformers do not require 4-wire input; a 3-wire primary feeder circuit suffices to supply a 4-wire secondary circuit. That is because any zero sequence current required by the secondary to supply earth faults or unbalanced loads is supplied by the delta primary winding, and is not required from the upstream power source. The method of earthing on the secondary is independent of the primary for delta-star transformers.
  • The second criterion to consider is what phase-shift you want between primary and secondary. For example, Dy11 and Dy5 transformers are both delta-star. If we don’t care about the phase-shift, then either transformer will do the job. Phase-shift is important when we are paralleling sources. We want the phase-shifts of the sources to be identical.
  • If we are paralleling transformers, then you want them to have the same the same vector group. If you are replacing a transformer, use the same vector group for the new transformer, otherwise the existing VTs and CTs used for protection and metering will not work properly.
  • There is no technical difference between the one vector groups (i.e. Yd1) or another vector group (i.e. Yd11) in terms of performance. The only factor affecting the choice between one or the other is system phasing, ie whether parts of the network fed from the transformer need to operate in parallel with another source. It also matters if you have an auxiliary transformer connected to generator terminals. Vector matching at the auxiliary bus bar

 Application of Transformer according to Vector Group:

(1)  (Dyn11, Dyn1, YNd1, YNd11)

  • Common for distribution transformers.
  • Normally Dyn11 vector group using at distribution system. Because Generating Transformer are YNd1 for neutralizing the load angle between 11 and 1.
  • We can use Dyn1 at distribution system, when we are using Generator Transformer are YNd11.
  • In some industries 6 pulse electric drives are using due to this 5thharmonics will generate if we use Dyn1 it will be suppress the 5th harmonics.
  • Star point facilitates mixed loading of three phase and single phase consumer connections.
  • The delta winding carry third harmonics and stabilizes star point potential.
  • A delta-Star connection is used for step-up generating stations. If HV winding is star connected there will be saving in cost of insulation.
  • But delta connected HV winding is common in distribution network, for feeding motors and lighting loads from LV side.

(2)  Star-Star (Yy0 or Yy6)

  • Mainly used for large system tie-up Transformer.
  • Most economical connection in HV power system to interconnect between two delta systems and to provide neutral for grounding both of them.
  • Tertiary winding stabilizes the neutral conditions. In star connected transformers, load can be connected between line and neutral, only if
    (a) the source side transformers is delta connected or
    (b) the source side is star connected with neutral connected back to the source neutral.
  • In This Transformers. Insulation cost is highly reduced. Neutral wire can permit mixed loading.
  • Triple harmonics are absent in the lines. These triple harmonic currents cannot flow, unless there is a neutral wire. This connection produces oscillating neutral.
  • Three phase shell type units have large triple harmonic phase voltage. However three phase core type transformers work satisfactorily.
  • A tertiary mesh connected winding may be required to stabilize the oscillating neutral due to third harmonics in three phase banks.

(3)  Delta – Delta (Dd 0 or Dd 6)

  • This is an economical connection for large low voltage transformers.
  • Large unbalance of load can be met without difficulty.
  • Delta permits a circulating path for triple harmonics thus attenuates the same.
  • It is possible to operate with one transformer removed in open delta or” V” connection meeting 58 percent of the balanced load.
  • Three phase units cannot have this facility. Mixed single phase loading is not possible due to the absence of neutral.

 (4)  Star-Zig-zag or Delta-Zig-zag (Yz or Dz)

  • These connections are employed where delta connections are weak. Interconnection of phases in zigzag winding effects a reduction of third harmonic voltages and at the same time permits unbalanced loading.
  • This connection may be used with either delta connected or star connected winding either for step-up or step-down transformers. In either case, the zigzag winding produces the same angular displacement as a delta winding, and at the same time provides a neutral for earthing purposes.
  • The amount of copper required from a zigzag winding in 15% more than a corresponding star or delta winding. This is extensively used for earthing transformer.
  • Due to zigzag connection (interconnection between phases), third harmonic voltages are reduced. It also allows unbalanced loading. The zigzag connection is employed for LV winding. For a given total voltage per phase, the zigzag side requires 15% more turns as compared to normal phase connection. In cases where delta connections are weak due to large number of turns and small cross sections, then zigzag star connection is preferred. It is also used in rectifiers.

(5)  Zig- zag/ star (ZY1 or Zy11)

  • Zigzag connection is obtained by inter connection of phases.4-wire system is possible on both sides. Unbalanced loading is also possible. Oscillating neutral problem is absent in this connection.
  • This connection requires 15% more turns for the same voltage on the zigzag side and hence costs more. Hence a bank of three single phase transformers cost about 15% more than their 3-phase counterpart. Also, they occupy more space. But the spare capacity cost will be less and single phase units are easier to transport.
  • Unbalanced operation of the transformer with large zero sequence fundamental mmf content also does not affect its performance. Even with Yy type of poly phase connection without neutral connection the oscillating neutral does not occur with these cores. Finally, three phase cores themselves cost less than three single phase units due to compactness.

 (6)  Yd5:

  • Mainly used for machine and main Transformer in large Power Station and Transmission Substation.
  • The Neutral point can be loaded with rated Current.

 (7)  Yz-5

  • For Distribution Transformer up to 250MVA for local distribution system.
  • The Neutral point can be loaded with rated Current.

 Application of Transformer according  according to Uses:

  •  Step up Transformer: It should be Yd1 or Yd11.
  • Step down Transformer: It should be Dy1 or Dy11.
  • Grounding purpose Transformer: It should be Yz1 or Dz11.
  • Distribution Transformer: We can consider vector group of Dzn0 which reduce the 75% of harmonics in secondary side.
  • Power Transformer: Vector group is deepen on application for Example : Generating Transformer : Dyn1 , Furnace Transformer: Ynyn0.

Convert One Group of Transformer to Other Group by Channing External Connection:

(1)  Group I: Example: Dd0 (no phase displacement between HV and LV).

  • The conventional method is to connect the red phase on A/a, Yellow phase on B/b, and the Blue phase on C/c.
  • Other phase displacements are possible with unconventional connections (for instance red on b, yellow on c and blue on a) By doing some unconventional connections externally on one side of the Transformer, an internal connected Dd0 transformer can be changed either to a Dd4(-120°) or Dd8(+120°) connection. The same is true for internal connected Dd4 or Dd8 transformers.

(2)  Group II: Example: Dd6 (180° displacement between HV and LV).

  • By doing some unconventional connections externally on one side of the Transformer, an internal connected Dd6 transformer can be changed either to a Dd2(-60°) or Dd10(+60°) connection.

(3)  Group III: Example: Dyn1 (-30° displacement between HV and LV).

  • By doing some unconventional connections externally on one side of the Transformer, an internal connected Dyn1 transformer can be changed either to a Dyn5(-150°) or Dyn9(+90°) connection.

(4)  Group IV: Example: Dyn11 (+30° displacement between HV and LV).

  • By doing some unconventional connections externally on one side of the Transformer, an internal connected Dyn11 transformer can be changed either to a Dyn7(+150°) or Dyn3(-90°) connection.

Point to be remembered:

  • For Group-III & Group-IV: By doing some unconventional connections externally on both sides of the Transformer, an internal connected Group-III or Group-IV transformer can be changed to any of these two groups.
  • Thus by doing external changes on both sides of the Transformer an internal connected Dyn1 transformer can be changed to either a: Dyn3, Dyn5, Dyn7, Dyn9 or Dyn11 transformer, This is just true for star/delta or delta/star connections.
  • For Group-I & Group-II: Changes for delta/delta or star/star transformers between Group-I and Group-III can just be done internally.

Why 30°phase shift occur in star-delta transformer between primary and secondary?

  • The phase shift is a natural consequence of the delta connection. The currents entering or leaving the star winding of the transformer are in phase with the currents in the star windings. Therefore, the currents in the delta windings are also in phase with the currents in the star windings and obviously, the three currents are 120 electrical degrees apart.
  • But the currents entering or leaving the transformer on the delta side are formed at the point where two of the windings comprising the delta come together – each of those currents is the phasor sum of the currents in the adjacent windings.
  • When you add together two currents that are 120 electrical degrees apart, the sum is inevitably shifted by 30 degrees.
  •  The Main reason for this phenomenon   is that the phase voltage lags line current by 30degrees.consider a delta/star transformer. The phase voltages in three phases of both primary and secondary. you will find that in primary the phase voltage and line voltages are same, let it be VRY(take one phase).but, the corresponding secondary will have the phase voltage only in its phase winding as it is star connected. the line voltage of star connected secondary and delta connected primary won’t have any phase differences between them. so this can be summarized that “the phase shift is associated with the wave forms of the three phase windings.

 Why  when Generating Transformer is Yd1 than Distribution Transformer is Dy11:

  • This is the HV Side or the Switchyard side of the Generator Transformer is connected in Delta and the LV Side or the generator side of the GT is connected in Star, with the Star side neutral brought out.
  • The LV side voltage will “lag” the HV side voltage by 30 degrees.
  • Thus, in a generating station we create a 30 degrees lagging voltage for transmission, with respect to the generator voltage.
  • As we have created a 30 degrees lagging connection in the generating station, it is advisable to create a 30 degrees leading connection in distribution so that the user voltage is “in phase” with the generated voltage. And, as the transmission side is Delta and the user might need three phase, four-wire in the LV side for his single phase loads, the distribution transformer is chosen as Dyn11.
  • There is magnetic coupling between HT and LT. When the load side (LT) suffers some dip the LT current try to go out of phase with HT current, so 30 degree phase shift in Dyn-11 keeps the two currents in phase when there is dip.
  • So the vector group at the generating station is important while selecting distribution Transformer.

Vector Group in Generating-Transmission-Distribution System:

  • Generating TC is Yd1 transmitted power at 400KV, for 400KV to 220KV Yy is used and by using Yd between e.g. 220 and 66 kV, then Dy from 66 to 11 kV so that their phase shifts can be cancelled out. And for LV (400/230V) supplies at 50 Hz are usually 3 phase, earthed neutral, so a “Dyn” LV winding is needed. Here GT side -30lag (Yd1) can be nullify +30 by using distribution Transformer of Dy11.
  • A reason for using Yd between e.g. 220 and 66 kV, then Dy from 66 to 11 kV is that their phase shifts can cancel out and It is then also possible to parallel a 220/11 kV YY transformer, at 11 kV, with the 66/11 kV (a YY transformer often has a third, delta, winding to reduce harmonics). If one went Dy11 – Dy11 from 220 to 11 kV, there would be a 60 degree shift, which is not possible in one transformer. The “standard” transformer groups in distribution avoid that kind of limitation, as a result of thought and experience leading to lowest cost over many years.

Generator TC is Yd1, Can we use Distribution TC Dy5 instead of Dy11.

  • With regards to theory, there are no special advantages of Dyn11 over Dyn5.
  • In Isolation Application: In isolated applications there is no advantage or disadvantage by using Dy5 or Dy11. If however we wish to interconnect the secondary sides of different Dny transformers, we must have compatible transformers, and that can be achieved if you have a Dyn11 among a group of Dyn5’s and vice versa.
  • In Parallel Connection: Practically, the relative places of the phases remain same in Dyn11 compared to Dyn5.
  • If we use Yd1 Transformer on Generating Side and Distribution side Dy11 transformer than -30 lag of generating side (Yd1) is nullify by +30 Lead at Receiving side Dy11) so no phase difference respect to generating Side and if we are on the HV side of the Transformer, and if we denote the phases as R- Y-B from left to right, the same phases on the LV side will be R- Y -B, but from left to Right.
  • This will make the Transmission lines have same color (for identification) whether it is input to or output from the Transformer.
  • If we use Yd1 Transformer on Generating Side and Distribution side Dy5 transformer than -30 lag of generating side (Yd1) is more lag by  -150 Lag at Receiving side (Dy5) so Total phase difference respect to generating Side is 180 deg (-30+-150=-180) and if we are on the HV side of the Transformer, and if we denote the phases as R- Y-B from left to right, the same phases on the LV side will be R- Y -B, but from Right to Left.
  • This will make the Transmission lines have No same color (for identification) whether it is input to or output from the Transformer.
  • The difference in output between the Dyn11 and Dny5 and is therefore 180 degrees.


About Jignesh.Parmar
Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical). He is member of Institution of Engineers (MIE) and CEng,India. Membership No:M-1473586.He has more than 16 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He has published numbers of Technical Articles in “Electrical Mirror”, “Electrical India”, “Lighting India”,”Smart Energy”, “Industrial Electrix”(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.

134 Responses to Vector Group of Transformer

  1. Shantanu Joshi says:

    In a system of 1 grid incomer connected to Dy11 transformer and a local generator connected to Dy1 transformer is it possible to synchronise the two incomers?

  2. Thank you so much for whatever you have, the best you are giving to us regarding the subject Transformers. Brother Jignesh, kindly provide a bit of ” ANSI Standard Transformer ” also in brief wherever it suits for the comparison of IEC Standards.

  3. nikhil says:


  4. Priyanka says:

    Thank you so much for this information

  5. Tapani Julin says:

    Six Ways to wire Star Winding is very helpful to me,Honestly i don’t know how to do it but because of it i learn thanks for a very good information.

  6. Byju Ramachandrn says:

    Dear Sir,

    I understood that the generator transformer vector group is delta at generator side and star in yard side. But in your article it is mentioned as reverse order. Please clarify

    • As per IEC Vector group shown in HV winding / LV winding so i Generator transformer HV side is Transmission side and LV is Generator side.

      • M.B.Sinha says:

        Dear Mr. Parmar, While I appreciate your article on significance and selection of vector group of Transformers, pl. let me know in a co-gen power plant where export/ import of power is envisaged, if the vector group YnD1 where in Generator Power Transformer’s secondary winding is Delta connected ( Generator side) and the distance of generator is within 100 meters from the stated transformer, is it necessary to install an earthing transformer? What would be the repercussion if the earthing transformer is not provided? I have installed and operated no. of such power plants with this vector group without an earthing transformer for several years and nothing abnormal noticed even on event of earth fault.
        Secondly, can we opt for YnYno Transformer for such application apart from the above widely used vector group X-mer which will be more economical too. Is there any constraint in Yy0 configuration? I’ll solicit your reply on both the cited points asap.

      • chai says:

        write clearly

  7. k mohana krishna says:

    Iam an electrical engineer working in APTRANSCO, Sir I want to know the operation of OLTC in 100 mva power transformer with tap connection. we have 25 nos of taps with 21st tap as normal if possible explain with sketch.

    Thanking sir

    K Mohana Krishna

    • Imranullah says:

      Please see the Rating & Diagram plate of that (100 MVA) transformer.

    • SRINIVASAN.R says:

      Details are available in OLTC make web site or search in google website address- typed as MR OLTC

    • Dear Krishna: The 21 being normal tap and having 25 taps is not unusual. the taps specified is dependent on expected voltage variation. Probably your supply must be going
      1.25% x 20 = 30 percent negative ( assuming @ 1.25% taps. it may be less. see the diagram)
      may I add that in understanding the principle of OLTC operation you have to remember that volts per turn has to be reatained for getting the set voltage on the secondary.iif V goes up v/turn increases. so secondary voltage will increase. to bring back the secondary voltage, introduce more turns. so v/turn is retained. accordingly you will know whetgher tap moves up or down

  8. rehan says:

    we good stuff i really liked that


    all the informations are 100% practical…..thank you very much sir

  10. randy addae junior says:

    I really like your diagrames and the explanations

  11. jaswant singh bhamra says:

    i really like your diagrames and the explanations
    jaswant singh bhamra senior test & commissioning sts switchgear uk

  12. R.K.SHARMA says:

    Information quite useful.Can we use YD7 in place of Yd1 generator transformer.

  13. ram says:

    good job sir

  14. Md. Anowarul Islam says:

    This is a fantastic tropics. But I want to know about the test methode and voltage relation of different type vector group.

  15. umar says:

    sir what is matching factor in differential protection

  16. Rashid says:

    Please give the detail procedure of vector group test with the example of.Dyn1


    Use full information….

  18. Ajay Shida says:

    hank you so much sir

  19. Vinoth says:

    Nice information

  20. Marios Moschakis says:

    Dear Jignesh,

    Thank you for the interesting information about the selection of transformer’s vector group.

    I would like to add another interesting point: The positive effect and other characteristics of transformers of certain vector groups on the mitigation of voltage sags. This can be done by the positive effect of transformers of vector group 1, 5, 7 and 11 (e.g. Dy1, Yd5 etc) on asymmetrical faults and especially as regards one-phase-to-ground faults, which are the most frequent faults. Moreover, it can be proved that two-phase, two-phase-to-ground and three-phase faults at the one side of such transformers give the same sag magnitude at the other side of the transformer under common assumptions used in short-circuit analysis. I have published a journal and a conference paper on that effect.

    Kind Regards,

    Dr. Marios Moschakis, Lecturer
    Department of Electrical Engineering
    Technological Educational Institute of Larissa, GREECE

  21. M.Ali Imran says:

    if i have a transformer of vector group Ynd11 on generating can i take a neutral on LV side?

  22. Siddavatam Nizamuddin says:

    This is a good ready reference regarding vector group and selection of various vector groups.
    I would like to ask one thing regarding distribution transformers. As you know the distribution system normally on HV side connected in parallel and LV side radial. That means the LV of two transformers will never be connected in parallel. In that case, can we connect the two distribution transformers with different vector group in parallel?

  23. sachin says:

    Dear sir,which vector group give highest secondary voltage?

  24. Bino says:

    what are the advantage of star star over delta star or vice

  25. sachidanand kumar says:

    Thanks, To clearify my problem of vector group…transformer?

  26. Roman says:

    I do not even know how I ended up right here, but I assumed
    this submit used to be good. I don’t know who you’re however
    certainly you’re going to a famous blogger when you are not already. Cheers!

  27. I read this article and i realy appreciate your efforts.Thanks for sharing this information.This is very helpful for people……

  28. patomchon L says:

    What ‘s the difference in diagram between the Dyn11 and Dy11

  29. sunil says:

    we want to step up 415volts to 11000volts for tunnel work. the transformer we have is star (LV) and Delta(HV). How to give earth fault protection?

    • Roop Bundele says:

      For Step up Transformer 415/11kV

      You would normally choose Yd11 or Yd1 or Yy0 or ( Yz1 or Yz11)( depending upon if it is likely to go in parallal with other existing supply at this voltage or further after steps up/down) instead of Dy11 or Dy1 for Earth fault Protection.

      As per IEC presentation of vector Group First letter represent High voltage winding

  30. harish patel says:

    dear sir will you explaine me formula kva=1.732*v*i*pf in which v=line voltage or phase voltage,i=line ampere or phase ampere,because as per my knownge v= avrage line to line voltage and i= avrage phase to phase current but i am facing problem from transformer manufacturer
    hope you do needful to me

  31. javad ahamad says:

    very nice information,but why we are sending with 30 deg lag in generation station??what is the point behind lagging while sending?

  32. Timika Laljie says:

    dear sir
    am using the PSS Sincal software, and i can’t figure out which group vector represents the single phase two winding transformer.

    please help

  33. very helpful, thank you!

  34. rajasekaran palani says:

    For a 5MVA, 0415/6.6KV, Whic
    h vector group I have to choose – Please advise

  35. natasha says:

    What are the only phase displacements in degrees with which it is
    possible to parallel the secondaries of two transformers?

    • boyd shinjabale says:

      thanks a lot my country Zambia uses DY 11 type of vector group on most of the distribution transformers

  36. Rags says:

    Does the downstream transformer vektor groups have an effect on fault currents. This is specifically as distance from the source and voltage levels have effect

  37. Sujit Sengupta says:

    Great work Jignesh, I would like to connect with you,
    Sujit Sengupta,

  38. anandkumar says:

    I learn more subject from you thanks

  39. sandeep says:

    Jignesh Sir, Thank u for good information.

    In star-star transformer how can we do vector group test??

  40. Fuad Arshad says:

    Thanku… I liked ur post


  41. Egbe, edigbo michael says:

    Iam impressed by the explicit information about the transformer , and i say congrat and keep it up.

  42. niranjansahu says:

    Jignesh,I am Niranjan Sahu, Retired DGM of Odisha Hydropower Corpn. But I have spent much time in field of distribution. Can u advise how to check Power theft of consumers by the distribution companies.

  43. surendar says:

    how can we select vector group for transformer?

  44. Ajay says:

    Sir,we are designing a 50MVA solar power plant in India.We need some clarification while selecting the transformers.First we are stepping up the voltage from 400V to 22KV,further we are stepping up to 110KV for transmitting.Here which type(Eg.Dy,Yy or Dy) we need to select for the above one.The answer will be useful for us to design.

  45. sagar says:

    why both the supply Star & delta required in variable frequency drive of model ACS 800 of ABB make?

  46. sagar says:

    what is the reason of using star & Delta both the supply in Variable Frequency Drive in model ACS 800 of make ABB

  47. Prashant says:

    How one can check the vector group of transformer by externally applying voltage?

  48. umesh says:

    sir kindly explain for transformer vector group Ii0

  49. Gajanan says:

    Sir, how we find out vector group from calculation, if vector group not mention on name plate?

  50. shivya says:

    How can we calculate the vector group?

  51. vadivel says:

    sir how saw the vector group in transformer winding.

  52. S.Parameswara Iyer says:

    Sir, Thanks for sharing valuable information

  53. Deepak says:

    Dear jignesh brother,

    Your electrical notes are very very helpful. And i need a help i searched some more related to transformer but not able to get exact application for dry type transformer, upto what ratings the dry type transformers are used and exact difference between oil immersed transformer and dry type transformer.

    Thanks in advance and hope in your notes

  54. SRINIVASAN.R says:

    Thank u so much for giving lot of information regarding VECTOR GROUP,its all very useful to me. Once again thanks to u.

  55. prashant says:

    it is very best web site

  56. kenan eli lyova says:

    a good literature for reading and assimilation

  57. pijush kanti dey (dhrua) says:

    what is the calculation of copper in HV & LV side without removal of the transformer, where only total weight & core weight found & also what will be the size of winding wire.

  58. nayeem says:

    hi i would like to know what are the possible vector group connections for parallel or synchronization of transformers.

  59. Manu Ram says:

    Very informative. I need guidance with regards to use of Generator Step-up Transformer(11/132KV), which are normally of Yd1 or Yd11 Vector group. Can the Transformers with vector group Dyn11 or YNyn0d11 be used? 132Kv system, with which the transformer is supposed to be synchronized, is with solidly grounded neutral. Thanks.

    Manu Ram

  60. Prabir Chandra Dutta says:

    Very very informative and useful. It would be better if PDF format is available .

  61. hari says:

    What are the conditions to confirm DY11?

  62. simon says:

    before testing the vector group of a transformer,how do i calculate the voltage between R-r in a star delta or delta-star

  63. lee says:

    sir, please recheck your drawings regarding transformer polarity, additive and subtractive polarity drawing had been interchanged. thanks.

  64. Nszbeer says:

    what is the vector group conditions for Dd0 Transformer?

  65. senthilmurugan says:

    in case of stepdown transformer [230/12],
    12 v ac is applied to secondary side. what will happen in primary?



  67. Heri says:

    Dear Sir,
    please give me advice
    we will synchronize generator with condition like below;
    – Transformer voltage is 70/6.6 kV ( LV side in Geneator and HV side in Bus)
    – Transformer vector is Yd11
    – Generator VT sensing on 6.6 kV
    – Bus VT sensing on 70 kV
    – VT sensing vector is Yy0

    so i will use the Phase sifter transformer after Bus VT for canceled out of phase shift.
    what the vector group of Phase shifter transformer that i need ?

    Thank you


  68. eknath wagh says:

    Dear Sir Myself Eknath, i was connecting three phase transformer in Y-Y configuration & observing both the primary & secondary waveform on Scope, both wear distorted sinewave, why i didnt got the reason, please suggest me how to resolve the problem

  69. Vikas Joshi says:

    I have a GSU trafo with YNd11 but the LV terminals are w v u from left to right instead of u v w required as per the Generator definition. The UAT (unit auxiliary trafo) on the generator busduct tap-of is Dyn11. My question is whether I need to match the u v w by doing interchange of phases in the busduct or I can connect generator u with GSU w and so on. In such case the GSU vector group will change or not. If yes than the what should be the vector group for UAT?

  70. Joe Witzel says:

    This is such a well written article. How can we get a copy to print out or in pdf?

  71. sudhir sharma says:

    Dear sir
    can you tell me procedure for finding vector group for unknown vector group of a transformer.

  72. User1 says:

    Hi Jignesh
    What would be the effects on the Zigzag neutral current during normal system operation, if the voltages are severely unbalanced due to load unbalancing on the power system feeders? Assuming Ynd1 Transformer with Zigzag connected on delta side for grounding purposes.

  73. ramkumar singh says:

    what is the differance vector group Dy11 and Dy5 of a transformer

  74. ali says:

    Dear Sir,
    Please describe which type of load is suitable for which vector group of transformer. i mean to say like VFDs, 3rd harmonic corresponds with DYN11 etc….

  75. Alan says:

    how to interchange phasing of 60 MVA furnace transformer of Dd0 connection.
    Thank You!

  76. David Payne says:

    Excellent article. However, you should cover autotransformers since 75% of the transmission transformer in North America are Y-Y autos with or without delta tertiaries or wih buried tertiaries.t

  77. Sir,
    Can u tell me the vector group connection of YnO …..

  78. Charles obi says:

    Thanks I learned a lot from the informations

  79. pradeep anand says:

    I want to know the details of DY5D0 with +7.5 degree phase shift

  80. SATHISH says:

    i have a Dd0 vector group transformer of 1:1 ratio , 380/380 volt … In this how to check the vector group test.

  81. Mohan Sahu says:

    thank you sir, it’s very helpful for us now i can debate with my seniors

  82. Suresh Mangalore says:

    We have a DYN5 Transformer which is used as a step down transformer (10.5 kV to 0.38 kV ) Can we use it as a step up transformer (0.38 kV to 10.5 kV ) ? The transformer is a stand alone and is NOT in parallel with any other device.

  83. ankush sontakke says:

    thanks sir for sharing knowledge . it is very helpful

  84. Balamurugan says:

    i have a YNd1 trafo stepping down 33kV to 6.6kV

    plz let me know what is the HV neutral conductor size i can arrive at?

  85. Ashwini ojha says:

    Now I can solve gate questions thanks

  86. I have doubt in your drawings for zigzag connection. Isn’t it supposed to be connected in subtractive polarity?

  87. why there is no 90 degree lag/lead between primary and secondary

  88. amit singh says:

    nice content…. thanks a lot sir..

  89. mohammad says:

    thanks for posting helpful information for transformer designer. could you please explain angle of D1Z.

  90. abdulwahid assafany says:

    Thanks Sir;
    Could you help me for my case. We have 5MVA YnD11 stepup transformer which is should be connected between Genset 400V and transfer it to 11000V, but we have also a DYn11 transformer at the other side on STS crane.
    Is it ok at connection, what about distribution and how to nuetral earting .. and what about the harmonics produced from crane is it cancelled or affect my Genset ?
    Please reply me
    Please reply me
    Thanks again

    Eng. Abdulwahid assafany, working at Hodiadah Seaport. Yemen

  91. Shashidhar C says:

    I would like to synchronise LT 415V Generator voltage driven by back pressure steam to a LT bus derived out of dyn11 transormer. The capacity of transformer is 1500kva,Turbine generator is 1MW/1250kva.
    I invite your posiblility and suggesstions from author and the group please

  92. darshit says:

    sir it is possible to connect yy0 and yy6 xmer in parallel ?

  93. Baz Lewis says:

    brilliant many thanks

  94. Santhosh kumar says:

    Hai everybody is anybody knows about the Yd11:40 and Yz0:20 vector group in VFD Transformer so please explain me that what is :40 & :20 is stantes for can any one can explain and I also if u have any related documents please send to my mail ID given

  95. T GUNASEKARAN says:

    Good job. This helpful for all electrical engineer. You done a very good job.

  96. Md.Siddique Shariful Islam says:

    Thanks Sir,
    We Need More…..

  97. Surendra Kr. Verma says:

    Thank you so much Jignesh sir,
    For sharing good article.

    i have a query regarding Dy connection.
    You stated that in Dy connection , y nullifies the 3rd harmonics.
    I request to you, please explain that point.

  98. Chandan says:

    Hai Parmar Sir your informations are very educative, basically Iam from instrumentation area. Can you please guide me how to calculate nominal maximum demand (NMD). Iam sizing transformer for MCC

  99. sandipan says:

    Sir its very helpfull.
    What is floating neutral and .. weather all neutral can be connected to grid ? or indIvisual earth pit required which are not connected to grid earthing.

    what is the concept of indivisual earh pit. please explain

  100. Narsing Kulkarni says:

    1)what is the basic reason to connect R of HV & r of LV together in vector group?
    2) What will happen if we don’t do shorted R-r while testing?

  101. M.Mohsin Ansari says:

    Thank you so much very important knowledge
    Regards,M.Mohsin Ansari

  102. Raj says:

    Dear Parmar ji, Could please provide the difference between YNd5 & YNd11 vector group of the transformers. In one of the project the Generator Transformer (156MVA) vector group is YNd5. So can we suggest to the client for YNd11. We have to justify this point in Technical & Commercial aspect. Seeking your please help us in this regard.

  103. Dhinesh says:

    Hello Mr.Jignesh.Parmar
    What is the Diffrent Isolator & Miniature Circuit Breaker And MCB Types

  104. naile says:

    What is the disadvantages of dyn5 and dyn 11 in parallel if I do some unconventional connections externally on one side of the Transformer, an internal connected Dyn11 transformer can be changed either to a Dyn7(+150°) or Dyn3(-90°) connection. ?

  105. Thiru says:

    hats off…!

  106. Goverdhan says:

    On a converter transformer name plate ,connection is written as Yd11z11:20z11:40y0z0:20z0:40.What does 11:20,0:20 mean in that?
    It is a 3 phase transformer with 6 secondary windings

  107. Swaraj says:

    GT: 13.8 Kv – 138 Kv Step-up YNd1
    Dist. Xmer: 138 Kv – 34.5 Kv Step-down Dyn1

    so the phase difference will be nullified or will add -60 Degree ?

    According to You:

    Normally Dyn11 vector group using at distribution system. Because Generating Transformer are YNd1 for neutralizing the load angle between 11 and 1.

    What do you mean by neutralizing the load angle between 11 and 1 ?

    Also what does this mean:

    If we use Yd1 Transformer on Generating Side and Distribution side Dy11 transformer than -30 lag of generating side (Yd1) is nullify by +30 Lead at Receiving side Dy11) so no phase difference respect to generating Side and if we are on the HV side of the Transformer, and if we denote the phases as R- Y-B from left to right, the same phases on the LV side will be R- Y -B, but from left to Right

    • Roop Bundele says:


      When GT: YNd1 ( Y represents HV winding) & Dist Tr: Dyn1, the phase difference will be nullified.

  108. palak ariwala says:

    why station transformer is Dyn11 and generating transformer is Ynd11??

  109. Karl W Nylén says:

    In this modern times is it need for sending power both ways in a transfomer. How will this be affected by the vector lag, in an Dyn11? My config. is a 400 V TN( primary connected) to common grid and my sync. gen 230V IT supplies (true the transf) to the grid from time to time. Will the vectors change from Dyn11 to Yd1 depending of the power direction?

  110. RK JAYASWAL says:

    Very useful.

  111. Fed says:

    I think in diagrams “SIX WAY TO WIRE STAR WINDING” (Y4 and Y8), (Y10 and Y2) must be swapped.

  112. Jimmy Henningsson says:


    I have one straight but not so simple question.
    Is it possible to build a transformer with the following spec:

    Primary: Standard 3 phase input 400VAC 50/60 Hz, Load need to be somewhat even (10%) between the 3 phases.
    Secondary: 2 phase output @ 180 degrees, 240V Phase/Phase. 50/60 Hz
    Power is approx 11 KVA.


  113. Arefin Chowdhury says:

    how to draw Dd6 group in multisim softower please urgent

  114. govindasamy adhikesavan says:

    Very useful

  115. Govindasamy Adhi Kesavan ABB Kuwait says:

    Very useful

  116. NANDKISHOR R. DHURI says:

    Dear Sir,
    I appreciate your article on significance & selection of vector group, I would like to know about current harmonic reduction in six pulse rectifier transformer. Generally we use Delta – Star Transformer (Dyn11). Star connection fed to 3ph full wave diode bridge. Electrolytic capacitors also connected across DC to minimize ripple voltage. In this configuration while using line choke or DC choke we get 30% current harmonics(5th & 7th harmonics) at input of Rectifier Transformer. 5th harmonic is dominant. As per your article you said if Dyn1 is used it will suppress 5th harmonic.
    Same way is it possible to suppress 7th harmonics also. by connecting transformer in some other connection. Because if it is possible it will reduce total harmonic distortion(THD i)
    Please suggest as early as possible.

  117. Rohit says:

    Dear Sir,

    I think, vector arrangement diagram shown in the article for the Dy5 and Dy7 need to change as it may require correction.

  118. Aditi says:

    As you said:
    We can use Dyn1 at distribution system, when we are using Generator Transformer are YNd11.
    Then why we always use YNd1 only for Generator Transformer?

  119. DNYANESM SHINDE says:

    Sir 69.69% mark means min 70% or not ans mi sir

  120. Muhammad Mohsin Ansari says:

    Thanks you so much it’s best best knowledge

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